The present invention relates to a writing instrument which employs a low viscosity ink such as a water-base ink.
Writing instruments using a low viscosity ink are provided with a pen member and an ink chamber, and are constructed such that the pen member and the ink chamber are interconnected through an ink passage for supply of ink in the ink chamber to the pen member; air in an amount equal to the amount of ink consumed is introduced into the ink chamber to thereby control the outflow of ink; and the ink extruded upon increase of the internal pressure of the ink chamber is stored in a storage portion and this stored ink is later consumed for writing to thereby prevent the ink from jetting to the exterior. As a typical example of such construction, there is known such a pen core mechanism of a fountain pen as disclosed in Japanese Patent Publication No. 7164/1967, in which when ink is extruded into a cavity portion of a pen core formed as an ink storage portion upon increase of the internal pressure of an ink chamber (ink tube), the ink stored in the cavity is only consumed by writing, and even when the internal pressure of the ink chamber (ink tube) decreases in a downwardly held state of pen, the above stored ink does not return to the ink chamber side because air enters the ink chamber through an air channel.
When ink stored in the above cavity is consumed, it is rarely the case that all the stored ink in the cavity is exhausted in a single writing use. Actually, ink remains in the above cavity in many cases. For example, in the case of a fountain pen having an ink chamber of 1 cc. or so, consumption of the total amount (1 cc.) of ink requires a writing distance of 500 to 1000 m. Consequently, assuming that about 10%, i.e. 0.1 cc., of ink was stored at a time within the above cavity at a temperature variation of 30.degree. C., its exhaustion by writing requires 50 to 100 m writing. Usually, however, the writing length with a fountain pen is shorter than 1 m, so in many cases ink always remains within the above cavity.
Therefore, with the conventional fountain pen based on the idea that the stored ink is exhausted by a writing use, ready for the next increase of the internal pressure, there occurs a case where ink is newly extruded upon the next increase of the internal pressure while the stored ink remains in the pen core cavity. If this occurs many times due to repeated temperature variations, the amount of stored ink gradually increases, resulting in leakage of ink, so-called ink drops. Particularly when a writing instrument is used for an automatic drawing machine or for a recorder, the aforementioned ink drops are symptomatic of a serious defect because the pen nib is always held in a downward state.